Mobile cranes have been designed with telescopic booms, and boom extensions, to handle heavier loads and raise those loads to greater heights. The load handling capacities of booms and boom extensions are increased to accommodate loads and longer extensions for reach. One of the ways to increase the load-handling capacity of a boom is to increase its cross sectional dimensions, and the thickness Of the materials from which it is made. As boom sections increase in size and material thicknesses increase, the booms can become heavier which may decrease lifting capacity. Various approaches in boom design and construction have been employed to achieve greater size and strength without adding undue weight. It is now common practice, for example, for lattice extensions to be attached to the tips of booms, to extend the effective reach of the boom without adding undue weight. In certain stationary and crawler type cranes, the support for the boom is also of lattice construction.
The principle elements of the lattice section of a crane are its chords, which run the length of the lattice section, and struts which tie the chords together. Typically, a lattice section will have three or four chords, and numerous struts.
In typical crane lifting operations, involving a crane having a lattice section or extension, a wire rope passes over a sheave at the end of the lattice section. The sheave supports the wire rope, which, in turn, supports a headache ball and load hook, from which a load may be suspended.
When a lattice section of a crane is dismantled from the lower boom section, then transported, sleeves are often used to protect the chords of the lattice section against abrasion. When a crane having a lattice section is in operation, however, a kind of damage significantly more severe than mere abrasion may happen to lattice section extension--e.g., a strut or chord may be impacted by a headache ball swinging from the end of its wire rope. Typically, sleeves Which are used to protect transported equipment are not designed to protect a lattice section from the severe damage that may be caused by the impact of a headache ball.
Although a damaged strut can often be repaired or replaced, the repair or replacement of a chord is often infeasible due to the chord's large size and many connections. If a crane operator uses a lattice. extension containing a damaged chord, the operator may risk both personal injury and property damage.
For the safety of the operator and others, and because of practical difficulties involved in the repair of a damaged chord, it is not uncommon for a manufacturer to recommend the abandonment of an entire damaged lattice section. When a lattice section has to be abandoned, it is generally costly in terms of lost time and money, and construction delays.